| The middle Majiagou f.m.Formation of Ordovician in the Eastern of Central paleo-uplift,Ordos Basin is the new horizon for gas exploration in this area. With the breakthrough of exploration to mid-formation stratum, the study of reserevoir of the middle Majiagou f.m.Formation is developed gradually. Based on the former researches, this thesis takes sedimentary petrology, sedimentary geochemistry and reservoir geology as its theoretical principles. It analyzes polystomatous drilling core, such as its description, sample analysis and scanning electron microscope. It also combines logging information and analysis of porosity and permeability to study the sedimentary facies, diagenesis and reservoir characteristics. In addition, the thesis tries to research into genetic model and reservoir controlling factor of the dolomite and gains much result on research horizon.The middle Majiagou f.m.Formation is hot, drought and ncrement of fresh water are small. In addition, its evaporation is strong which makes the sea of survey region be a low energy depositional environment of shallow water, high salinization and easily exposure It also develops supralittoral zone,intertidal zone and sublittoral zone. Based on the lithologic assemble, sedimentary facies is ulteriorly divided into five sedimentary micro-facies.To combine the observation of thin slice with cathodeluminescence and scanning electron microscope, diagenetic stage is divided into four strike-slips in total. The main diagenesis types of middle Majiagou f.m.Formation are dolomization, compaction, pressure solution, recrystallization, dissolution and removing ointment effect. Among them, dolomization, removing ointment effect and dissolution are good for formation and development of crack. They are constructive diagenesis.The extent of porosity of the middle Majiagou f.m.Formation in research region is about 2-8%, and the research region belongs to low permeability layer. The crack types of the middle Majiagou f.m.Formation are intercrystal porosity, bird's-eye pore, plaster mole hole intercrystal dissolution pore, dissolution pore, dissolution corrosion and so on. Among them, dissolution pore, dissolution corrosion and intercrystal porosity are the main reservoir space type in the research region. The reservoir of powder crystal dolostone is the most advanced one. The secondary porosity, secondary lacune and secondary hole which are formed in exodiagenetic stage and buried diagenetic stage are the key factors which can form good reservoir or not. In addition, the development of structural fracture and how well its preservation also is the key factors. The subsequent structural fracture improves the storage-percolation system. By the geochemical prospecting method of X-diffraction, trace element and carbon and oxygen isotope, the genetic model of dolostone in research region is divided into two kinds: penecontemporaneous dolostone model and burial dolostone model. Compared with penecontemporaneous dolostone, burial dolostone has rough kernel, high degree of order and high extent of porosity. It also has the characteristics of low Mn and Fe, high Sr, ow C and O isotope. Crystal grain dolostone by burial genetic cause of formation becomes the main lithology of dolostone reservoir. However, the storage of gas reservoir in research region is directly connected with the thickness of dolostone. In addition, precipitation facies, diagenesis and tectonization control together, and precipitation micro-facies and diagenesis are key factors to the development of good reservoir. However, the perfect source-reservoir-seal assemblage is essential condition of deposits. |
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